Why do you think the border states chose to remain in the Union despite their support of slavery?

First, they were all one nation, and every problem could be resolved in some way, and the South had some good crops, technologies, and other things the north didn't have, such as fertile rivers perfect for farming. Plus, the North though it was unconstitutional for the South to secede from the Union, so that was another reason the North wanted the south to say with the Union.